1. Field of the Invention
The present invention relates to tracking technology used in read-only optical disks such as CD-ROMs and/or DVDs etc., wave-form equalization technology for address recognition and data synchronizing technology.
2. Description of the Related Art
In recent years, the speed of optical disk drive apparatus has continued to increase. In this connection, important roles are played by tracking techniques for achieving high speed access and wave-form equalization for address reproduction.
A prior art optical disk drive apparatus is described below with reference to the drawings. FIG. 9 is a block diagram of a prior art optical disk drive apparatus. In FIG. 9, 101 is an optical disk constituted by a CD (compact disk) or DVS (digital video disk); this is mounted on a spindle motor. 102 is an optical head, which is provided with a laser light source and an orthogonally divided photosensor 121. A laser beam reflected by the optical disk information recording surface is directed on to the orthogonally divided photosensor, and electrical signals are output from the respective sensor elements. Of these output signals, signals from sensor elements that are located mutually opposite in the diagonal direction are added to each other and supplied to phase error detector 105. Positive and negative phase differences are generated in accordance with the direction and degree of the tracking error (offset of the focal point of the laser beam from the centre line during tracking) of these diagonally added signals. Viewing the outputs of these phase error detectors through filter 106, a tracking error signal TE whose polarity and amplitude changes in accordance with the tracking error is obtained. The tracking error signal is fed back to a tracking actuator of optical head 102 and tracking control is thereby performed (see for example Japanese Patent Publication No. H.5-80053).
However, with this system (differential phase tracking detection), tracking offset is easily generated due to the effect of aberration of the optical head and/or disk tilt from the optical head optic axis of the optical disk. Specifically, due to such spatial factors, phase offset is generated in the diagonal addition signals, and this appears as a spurious tracking error signal.
A delay element 103 and variable means for delay 104 are therefore employed. Means for delay 103 generates a time delay .tau. and variable means for delay 104 generates a time delay within a range of 0.about.2 .tau. in accordance with a set value d that is supplied by a controller 109. A time difference is thereby generated that cancels the phase offset caused by the spatial factors, and it is believed that this offset or, in other words, tracking error component, can thereby be cancelled.
However, with the prior art, when the linear velocity of the optical disk recording information changes, the balance of the offset and compensation amount is lost, causing the problem that offset fluctuation is produced.
Specifically, the reason for this is that, whereas the phase distortion is generated by spatial factors such as the aberration of the optical system, the correction is implemented time-wise under the assumption that the wave form is generated with constant velocity. Essentially, even if the phase distortion is the same, if the linear velocity is high, tracking offset must be corrected with a small time error, while, if the linear velocity is small, correction must be effected with a large timing error.
This was scarcely a problem in the conventional case where CDs or DVDs were reproduced with predetermined linear velocity for audio or video applications, but in future the linear velocity will often be frequently changed when these are used in computer applications. An example that may be given is CAV (constant annular velocity) reproduction. The original CDs or DVDs were for reproducing music or video and were manufactured under the assumption that the information would be reproduced with constant linear velocity (CLV). Specifically, when such disks were cut, the master disk was rotated with fixed linear velocity and digital information was recorded with a fixed transfer rate. Consequently, when these were reproduced, the spindle motor was controlled so as to be driven with a fixed linear velocity irrespective of the speed of the inner or outer periphery of the disk, by varying the speed of rotation of the spindle motor at the inner or outer periphery of the disk. However, when these are employed as so-called CD-ROMs or DVD-ROMs for data files of computers, since random access is performed over both the inner and outer peripheries of the disk, attempting to reproduce the information with fixed linear velocity results in the spindle motor being repeatedly frequently accelerated and decelerated. As a result, it is difficult to raise the access speed of the data and furthermore power is consumed every time acceleration/deceleration is carried out.
Recently therefore, CD-CAV reproduction or DVD-CAV reproduction has been proposed in which the information is reproduced with the spindle motor always being rotated at constant angular velocity. Of course the transfer rate of the information signal at the inner and outer peripheries of the disk is different, but this can be absorbed by buffer memory, so, from the point of view of information signal processing, scarcely any problems arise. It does however result in offset of the time axis compensation when the tracking compensation described above is performed.
Also, apart from the above tracking offset compensation, time-wise compensation in respect of spatial factors could be performed using for example a wave-form equalizing filter. Specifically, when reproducing information from a carrier on which information is recorded with high density, such as a DVD, accurate reproduction cannot be achieved unless a filter is employed in which the gain in the higher frequency band is raised in accordance with a predetermined cut-off frequency. However, in the case of CAV reproduction as described above, there is a relative offset between the cut-off frequencies at the inner and outer peripheries of the disk. This presents a problem in particular when reproducing an address immediately after a track jump. Specifically, the method has been considered for example of seeking the optimum value of the equalization filter cut-off frequency such as for example to minimise the error of the information reproduction signal, if a sufficient time margin is available, but this suffers from the problem that time is required for track access (time from the start of the track jump until the information region of the desired address is reached).
The same problem arises if a PLL (phase locked loop) is used to synchronize the clock to the reproduction signal that is read from the carrier. Specifically, while the object of a PLL is to suppress frequency fluctuation of the reproduced signal i.e. jitter, in the case of CAV reproduction, since the linear velocities are different respectively at the inner and outer periphery of the disk, the centre frequency of jitter changes in proportion thereto, giving rise to the problem that optimum gain of the PLL cannot be set.